1. Field of the Invention
The present invention is directed to a process for preparing continuous strip cast aluminum alloy suitable for use in the manufacture of deep drawn and wall-ironed articles such as cans and the like.
2. The Prior Art
In recent years, aluminum alloys such as the Aluminum Association specification 3004 have been successfully fabricated into two piece beverage cans by deep drawing and ironing. The expanding use of two piece aluminum cans has created a need for aluminum alloy sheet for forming the can body that not only possesses the required combination of formability and strength properties but is also economical to manufacture.
Typically the aluminum alloy sheet useful in the production of deep drawn and ironed beverage cans is cast by direct chill casting an ingot having a thickness of about 20-25 inches. The ingot is homogenized at 950.degree.-1125.degree. F. for 4-24 hours and then subjected to hot rolling wherein the ingot is passed through a series of breakdown rolls maintained at a temperature of 400.degree.-900.degree. F. to reduce the ingot in thickness to a reroll gauge of about 0.130 inch.
Thereafter the reroll stock is subjected to an annealing step wherein the stock is heated at 600.degree.-900.degree. F. for 0.5-3 hours to effect recrystallization of the metal structure. The annealed reroll stock is subjected to a final work hardening step wherein the reroll stock is cold rolled (room temperature rolling) to a final gauge of about 0.013 inch or about 90% of its original thickness to produce the substantially full hard (H19) temper required for two-piece can body stock.
In spite of the successful use in can-making of direct chill ingot cast aluminum alloy, economic and energy considerations would favor the manufacture of the aluminum sheet by continuous strip casting. In this process the molten aluminum is cast and solidified into a thin web of one inch or less in thickness so that subsequent rolling is reduced and the costly step of hot rolling is eliminated.
In the manufacture of continuous strip cast aluminum alloy for can manufacture, the thin, e.g., 0.2-1.0 inch solidified cast web is typically reduced in thickness to a gauge of about 0.130 inch by cold rolling with an intermediate recrystallization anneal at about 600.degree.-900.degree. F. Thereafter, as in the manufacture of direct chill ingot cast stock, the thinned, annealed stock is subjected to a final work hardening step by cold rolling to a final gauge of about 0.013 inch to produce the H19 temper required for can body manufacture.
Although the continuous stip cast aluminum alloy is advantageously utilized for many fabricated products, such stock has not been used extensively for the manufacture of drawn and wall-ironed can bodies.
In the production of two-piece drawn and wall-ironed beverage cans, circular discs or blanks are cut or punched from the cold worked (H19) sheet for deep drawing into the desired shape. Deep drawing is a process for forming sheet metal between punch and die to produce a cup or shell-like part. When a deep drawn shell with a heavy bottom and thin sidewalls is desired, wall-ironing is used in conjunction with deep drawing. The blank is first drawn to approximately the final diameter cup. The sidewalls are then reduced in thickness in one or more ironing operations.
Because of the nature of the working stresses incurred during wall-ironing of the deep drawn shell, when continuous strip cast aluminum alloy such as 3004 is subjected to wall-ironing, scoring may occur on the die surface; alternately, deep grooves may appear on the finished can which is referred to in the art as "galling". Galling adversely affects the acceptability of the can product and the effectiveness of the can manufacturing process. Galling is not normally observed during wall-ironing aluminum sheets of the same alloy composition produced from direct chill cast ingot.
In spite of the economic advantage of the strip casting process, due to the drawback of not being gall-free when subjected to severe mechanical operations such as wall-ironing operations in two-piece aluminum can making, the utility and applicability of continuous strip cast aluminum alloy for can making has been extremely limited.
The art has addressed the problem of providing continuous strip cast aluminum alloys which have the capability to be gall-free when subjected to the severe mechanical working conditions of can making. For example, U.S. Pat. No. 4,111,721 discloses a process for imparting an anti-galling character to continuous strip cast aluminum alloy wherein the aluminum strip is heat treated at a temperature of at least 900.degree. F. and advantageously at about 1150.degree. F. for a period of time between about 16 to 24 hours prior to its final cold reduction pass.
The art prior to U.S. Pat. No. 4,111,721, namely U.S. Pat. No. 3,930,895 disclosed that in the process of making continuous strip cast aluminum alloy suitable for can making, the cast strip, before cold rolling, is homogenized at a temperature of about 950.degree. to 1050.degree. F. for about 8 to about 16 hours.
Although the art reported that gall-free continuous strip cast aluminum alloy had been produced, the strip has remained substantially unacceptable for can making stock because of the problem of "earing" which manifests itself as a scalloped appearance around the top edge of the cup during the deep drawing cup formation step of the draw and wall-iron processing of the aluminum sheet.
The scallops, or ears, represent an almost universally undesirable feature of the cup as the ears must be removed in order to present a smooth or flat upper lip on the cup. This of course necessitates cup trimming prior or subsequent to wall-ironing, with an attendant increase in production costs and material waste.
The level of earing in a drawn cup is determined by the following equation: ##EQU1## where he is the distance between the bottom of the cup and the peak of the ear and ht is the distance between the bottom of the cup and the valley of the ear.
To be acceptable for can making, the aluminum alloy sheet when processed into a cup must exhibit a level of earing of no more than about 3.5% and preferably less than about 3% earing. The level of earing experienced with commercially available continuously cast strip of 3004 aluminum alloy is generally in the range of 5% or more.
It is evident, therefore, that the reduction of the degree of earing during deep drawing of continuous cast aluminum strip to a level of about 3.5% or less represents a major contribution to the art of manufacture of continuous cast aluminum strip can stock.